MoTeC Global Forum

Hi all trying to sort out an issue with voltage spike on my fuel pressure sensors..I have a drag race car with C125 dash/logger.. I run three pressure sensors, one for the engine fuel system and two for the nitrous system. The problem is soon as I launch the car the fogger pressure sensor spikes out voltage and peaks. It’s a 15 psi sensor and Max’s out. I’ve swapped over the sensor to 100 psi sensor (pump only puts out 30psi) and the same result happens and as you can see from the graph, it’s fine before I launch the car. Soon as I launch it peaks then as soon as I come off the throttle at the end of the track it drops back down to normal pressure for this few seconds that I leave the nitrous system on… as you can see from the graphs. desperately trying to get sorted as it’s the most important sensor on the car for me thanks in advance..green is fogger pressure, red is where I swapped plug over for another sensor. and pls excuse the bigger pictures can’t seem to make them smaller

Are you sure you don't have a mechanical wiring problem (like the throttle linkage is causing the fogger pressure signal to short with the 5V supply)?

Can you share a logged data file. Your screen shots don't allow much investigation.

Thanks for the reply David, and yep your suggestion sounds quite logical, it’s a solid throttle linkage and it not near the sensor wires, I will investigate that though, not quite sure how I share my logged data file I will have a go.. thanks again..

David Ferguson wrote:Are you sure you don't have a mechanical wiring problem (like the throttle linkage is causing the fogger pressure signal to short with the 5V supply)?

Can you share a logged data file. Your screen shots don't allow much investigation.

file attached i think?

That is the index fine (.ldx), need the file with the same name but ends in .ld

You may need to configure "folder options" to show file extensions.

think thats it ? thanks again david

Hi,

I have duplicated the channels that you are having issues with and set them up to monitor the Pin voltages being seen, I have also added them into the logging.

Any chance that AV4 really wired to a throttle position sensor? It sure looks like that, particularly as you back off at the end of the run.! Or the Fogger fuel pressure is actual higher than 15 psi when on full throttle?

I notice in the dash config, that you are using absolute voltage for your pressure sensor calibrations, normally pressure sensors are ratiometric (so their output scales with the 5V voltage source), I have changed all your sensors to this type. You should verify that your sensors are being supplied the correct voltage and have a 0.5V to 4.5V output. If they were ratiometric and being supplied 12V they would exceed the voltage limits of the channels AV1-4 on the dash.

You can log the voltage from the sensor by duplicating the channel, so I did this for the Fogger (creating Fogger Sensor Voltage), and added that channel, and the 5V channel to the logging. Here is your modified C125 config with these changes.



Also, your rear wheel speed calibration seems out, perhaps the wrong number of trigger teeth?

Perhaps you can test this even without the engine running, just applying full throttle and monitoring the sensor voltage.

Good Luck in your search...
(I see Stephen was on this as well.. Try his config...)

Thank you so much for this David, I bought the dash of a friend and yes one of the channels was a throttle position sensor, It could well have been the channel I’ve re calibrated to the fogger sensor, iam pretty new to this and not that great at the computer side of things but trying my best..lol.. I’ll try all of the above that you have suggested and hopefully get to the bottom of this.. got to say thanks again, this forum has been brilliant all round for me, you are all so helpful and much appreciated

David Ferguson wrote:Any chance that AV4 really wired to a throttle position sensor? It sure looks like that, particularly as you back off at the end of the run.! Or the Fogger fuel pressure is actual higher than 15 psi when on full throttle?

I notice in the dash config, that you are using absolute voltage for your pressure sensor calibrations, normally pressure sensors are ratiometric (so their output scales with the 5V voltage source), I have changed all your sensors to this type. You should verify that your sensors are being supplied the correct voltage and have a 0.5V to 4.5V output. If they were ratiometric and being supplied 12V they would exceed the voltage limits of the channels AV1-4 on the dash.

You can log the voltage from the sensor by duplicating the channel, so I did this for the Fogger (creating Fogger Sensor Voltage), and added that channel, and the 5V channel to the logging. Here is your modified C125 config with these changes.

Outlaw modified C125 Config.c125v64

Also, your rear wheel speed calibration seems out, perhaps the wrong number of trigger teeth?

Perhaps you can test this even without the engine running, just applying full throttle and monitoring the sensor voltage.

Good Luck in your search...
(I see Stephen was on this as well.. Try his config...)

Thank you Stephen much appreciated, I will check that out

Stephen Dean wrote:Hi,

I have duplicated the channels that you are having issues with and set them up to monitor the Pin voltages being seen, I have also added them into the logging.